According to the current state of the art, electrical energy can only be stored in a very limited and short term, which is why an electrical energy storage device is almost always referred to as a storage device that converts electrical energy into a form of energy that is easier to store and later converts it back into electricity. Bringing all electrical storage devices into one definition therefore presents legislators with challenges.
For this reason, the definition of energy storage systems was only amended in June 2022, as part of the Easter package, and laid down in the Energy Industry Act.
Energy Industry Act (EnWG) §3 15d (since June 22)
Energy storage system:
"Installation in an electricity network which postpone the final use of electrical energy to a later date than its generation, or which converts electrical energy into a storable form of energy, stores such energy and then converts it back into electrical energy or use as another energy source".
In short, an energy storage device is therefore a system which receives energy in the form of electricity from the power grid, possibly converts it into another form of energy, and then feeds it back into the power grid in the form of electrical energy with a time delay.
An energy storage device is therefore not an (end) consumer, since the electricity consumed is not consumed itself, but also not a producer, since the energy was not produced by the storage device itself.
An energy storage device is therefore only a buffer for the temporary storage of energy.
As a result of the continuous shutdown of large conventional power plants such as coal, gas or nuclear power plants and the construction of several million renewable generation plants, both the complexity and the volatility of the German power grid are increasing immensely.
There are already more and more days when the price of electricity falls into the negative, especially when a significant surplus is produced by PV & Wind during the day and prices rise massively again in the evening and expensive conventional power plants have to be started up to meet demand at nightfall.
More details about this can be found in the article"Large-scale battery storage as a key technology for the energy revolution" written.
In addition to these short-term fluctuations within a few hours, there is the dependence on weather and the seasonal dependence of renewable sources. Even with a week of cloudy and windless weather or a significantly lower production output of all PV systems in winter, the German power grid must be able to combine power generation and electricity consumption and thus ensure security of supply.
Achieving an energy revolution just by expanding renewable energy sources is therefore not possible and must be supplemented by an equally rapid expansion of both short-term and long-term energy storage systems, which make the power grid more flexible and stable and thus prepare it for the high number of volatile plants. This is shown, among other things, by the Fraunhofer study "Paths to a Climate-Neutral Energy System" of 12.11.21, which expects an enormous increase in electrical energy storage systems. (Battery storage increased from 0.45 GWh in 2021 to 83 GWh in 2030)
The different types of energy storage systems differ fundamentally in which form of energy the electricity is stored. In general, a distinction is made between chemical, electrochemical, thermal, mechanical and electrical energy storage devices.
Depending on the type of storage device and its properties, there are very different applications for energy storage devices. Some storage systems are better suited for short-term storage of energy due to high efficiencies and fast response times, while others are significantly more effective for long-term storage.
Since both short-term and long-term energy storage systems will be required to offset the volatility of renewable sources, the three energy storage systems are compared below, which are the current pioneers in the field of electrical energy storage due to their potential capacity and technical maturity.
Large battery storage systems are currently the best way to compensate for short-term volatility (e.g. day-night volatility). In addition to the highest efficiency, scaling battery storage systems is significantly easier than that of pumped storage power plants. Pumped storage power plants have high geographical requirements, as they must be built at a location with a large difference in altitude. In addition, the possible locations for pumped storage often also pose risks for nature conservation, which can make the planning and implementation of such projects significantly more difficult. Large-scale battery storage systems, on the other hand, are geographically independent and can be built anywhere. Due to the high energy density, land use is low and there are no significant environmental risks.
In addition, the construction of a pumped storage power plant can take 5-10 years, while a large battery storage system can be connected to the grid within 1-2 years. Since the expansion of energy storage systems must be massively accelerated over the next few years in order to achieve the goal of 80% electricity from renewable sources, the implementation time of the storage systems is a critical point.
When it comes to long-term storage of electricity, hydrogen is currently a technology with particularly high potential. Although the efficiency of electrolysis and fuel cells is significantly lower than that of a large battery storage or pumped storage system, the hydrogen can be stored for a long time in suitable storage media (e.g. LOHC technology or in the natural gas network). In this way, the seasonal fluctuations of renewable sources can also be balanced out.
There is agreement among experts that both short-term and long-term storage on a very large scale are needed to overcome key challenges of the energy transition - to compensate for short-term, day-night volatility fluctuations and to hedge against seasonal fluctuations. There is therefore no question of "large battery storage, pumped storage or hydrogen?" Rather, the energy system of the future requires "both and."
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